Isa 10:20-22, Isa 37:31-32, Zech 13:8-9, Rom 9:27-29
In the Old Testament, it was foretold that only a remnant of Israel would return to God. (Isa 10:20-22, Isa 37:31-32, Zechariah 13:8-9)
Isa 10:20 And it shall come to pass in that day, that the remnant of Israel, and such as are escaped of the house of Jacob, shall no more again stay upon him that smote them; but shall stay upon the Lord, the Holy One of Israel, in truth. 21 The remnant shall return, even the remnant of Jacob, unto the mighty God. 22 For though thy people Israel be as the sand of the sea, yet a remnant of them shall return: the consumption decreed shall overflow with righteousness.
Isa 37:31 And the remnant that is escaped of the house of Judah shall again take root downward, and bear fruit upward: 32 For out of Jerusalem shall go forth a remnant, and they that escape out of mount Zion: the zeal of the Lord of hosts shall do this.
Zechariah 13:8 And it shall come to pass, that in all the land, saith the Lord, two parts therein shall be cut off and die; but the third shall be left therein. 9 And I will bring the third part through the fire, and will refine them as silver is refined, and will try them as gold is tried: they shall call on my name, and I will hear them: I will say, It is my people: and they shall say, The Lord is my God.
Paul quotes the Old Testament and says that only a remnant of Israel will be saved. (Rom 9:27-29)
Rom 9:27 Esaias also crieth concerning Israel, Though the number of the children of Israel be as the sand of the sea, a remnant shall be saved: 28 For he will finish the work, and cut it short in righteousness: because a short work will the Lord make upon the earth. 29 And as Esaias said before, Except the Lord of Sabaoth had left us a seed, we had been as Sodoma, and been made like unto Gomorrha.